If A × B points along positive z-axis then the vectors A and B must lie in:

Explanation

(a) YZ-plane: If the cross product of vectors A and B points along the positive z-axis, it means that A and B must be perpendicular to the z-axis. In the YZ-plane, the x-coordinate is always zero, so if A and B lie in the YZ-plane, their x-components would be zero. This means that the cross product of A and B would not point along the positive z-axis, so option (a) is incorrect. (b) ZX-plane: Similarly, if A and B lie in the ZX-plane, their y-components would be zero. Again, this would result in the cross product not pointing along the positive z-axis, so option (b) is incorrect as well. (c) XY-plane: If A and B lie in the XY-plane, their z-components would be zero. This means that the cross product of A and B would only have a non-zero z-component, pointing along the positive z-axis. Therefore, option (c) is correct. (d) None of the above: This option is incorrect because one of the options must be correct based on the given condition. So, the correct option is indeed (c) - XY-plane.